There are desserts where subtlety is what counts. Where faint accents of flavors and delicate ingredients softly win the day. This, friends, is not one of them.
This is caramel, nearly burnt, taken to its darkest extreme. It's melt-in-your mouth duck fat, buttery, savory, and sweet all at once. And it's the bracing punch of soy sauce, salty enough to cut through all the sweetness and electrify the palate. It demands a love for savory desserts and duck fat. If you're not that type, it's not too forgiving.
But if this sounds like something up your alley, you can look forward to what may be the richest caramel you will ever eat. Besides adding complex flavor, the duck fat lets the caramel linger on the tongue long after the ice cream is gone. Soy sauce works surprisingly well with caramel, drawing out its buttery qualities.
It's hard to have more than a scoop at a time, so try cutting it with some pound cake or baked apples; the ice cream melts all too readily into sauce. The bottom line here? We get one life to live, and it's too short to not act out on wild impulses from time to time. This over-the-top ice cream doesn't take prisoners, and it won't kill you softly. But I wouldn't want it any other way.
In a heavy 3 quart saucepan, combine sugar with just enough water to barely moisten it. Place on high heat and let sugar melt and begin to caramelize. As sugar caramelizes, the bubbles will shrink and pile on each other. The caramel is nearly ready when bubbling significantly slows and the syrup is the color of an old penny, a dark amber. Do not stir the pot at this point.
Caramel will begin to smoke; let it for three seconds and then stir in half-and-half and turn off heat. If caramel seizes, heat on low and stir till dissolved.
In a large bowl, whisk together egg yolks, duck fat, and soy sauce till thoroughly combined. Ladle in about 1/3 of the caramel mixture, whisking constantly, to temper eggs. Then return egg mixture to saucepan. Heat custard on low, stirring frequently, until custard coats the back of a spoon and a finger swipe leaves a clean line. Remove from heat, stir in vanilla, and transfer to an airtight container. Chill overnight in refrigerator.
Custard may curdle slightly overnight. Pour through a fine-mesh strainer into an ice cream maker and churn according to manufacturer instructions. Chill in a cold freezer, 3 hours for a soft serve and 8 to 12 for a firmer ice cream. Sprinkle with a pinch of freshly grated nutmeg just before serving. Ice cream will melt quickly once scooped.